Python 2.7 quick reference

18. None: The special placeholder value

Python has a unique value called None. This
special null value can be used as a placeholder, or to signify
that some value is unknown.

In conversational mode, any expression that evaluates to None is not printed. However, if a value of None is converted to a string, the result is the string
'None'; this may happen, for example, in a print statement.

>>> x = None
>>> x
>>> print x
None

The value None is returned from any function
that executes a return statement with no value,
or any function after it executes its last line if that last
line is not a return statement.